Centrifugal mold for the casting of liquid metal and the process for producing said centrifugal mold

ABSTRACT

Centrifugal mold for casting liquid steel. The centrifugal mold is of low-alloy steel selected from the range of air-hardenable steels and having 0.15-0.30 percent of carbon and additions not exceeding 5 percent of the total. This steel has, throughout the thickness of the centrifugal mold, a regular and uniform structure in which at least 60 percent of the carbon is fixed in the form of chromium-and manganese-saturated cementite.

United States Patent 72] Inventor Eugene Herzog Nancy, France [21] Appl. No. 749,411

[22] Filed Aug. 1, 1968 [45] Patented Oct. 26, 1971 [73] Assignee Centre De Recherches De Pont-A-Monsson Pont-A-Monsson, France 32 Priority Aug. 8, 1967 [33] France [54] CENTRIFUGAL MOLD FOR THE CASTING OF LIQUID METAL AND THE PROCESS FOR PRODUCING SAID CENTRIFUGAL MOLD 10 Claims, 8 Drawing Figs.

[52] US. Cl 148/2,

75/124, 75/126 B, 75/126 C, 75/126 D, 75/126 E, 75/126 F, 75/126 P, 75/126 J, 75/128 A, 75/128 F, 75/128 N, 75/128 G, 75/128 Z, 75/128 T, 75/128 W, 148/36, 148/143, 164/286 [51] Int. Cl C21d 9/08, C22c 39/26 [50] Field ofSearch 148/2,3, 36, 31, 134, 143, 144; 75/124, 126, 128; 164/82,

[56] References Cited UNITED STATES PATENTS 2,572,191 10/1951 Payson 75/126 2,770,563 1 H1956 Herzog 148/36 2,861,908 11/1958 Mickelson et a1. 148/36 3,288,600 ll/1966 Johnsen et a1. 75/126 FOREIGN PATENTS 517,118 l/1940 Great Britain 75/126 751,987 7/ 1956 Great Britain 75/ 126 217,646 1 1/1968 U.S.S.R. 148/36 OTHER REFERENCES German Publication $25435 V1a /18d Patentanmeldung, Sept. 22, 1955 75/126 Primary Examiner-Charles N. Lovell Attorney-J. Delattre-Seguy saturated cementite.

The present invention relates to molds for the casting of liquid metal, and in particular to molds for the centrifugal casting of iron pipes or other parts of revolution of metals or metal alloys having a melting point lower than l,300 C.

It is known that the molds which are in direct contact with the liquid metal when casting, are subjected to very high thermic stresses due to the difference in temperature between their inner and outer faces. These stresses have values on the inner face of the mold exceeding 100 daN/mm.. They result in the formation of cracks which develop in the course of successive castings and markedly shorten the service life of the molds.

Further, in the utilization of the centrifugal casting molds a rapid cooling of the cast parts is desired and for this purpose the shells have the outer face cooled by a spraying of water, whereas the inner face, which is exposed to the liquid metal, is brought to a temperature of about 650 C., with a maximum of 700 C. It has been observed that under these conditions and with steels of known type the inner wall of the mold undergoes variations in structure in the course of the thermic cycles imposed by the casting. These structural changes attributed to the alignment and coalescence of the carbides (that is, the growth of the grains which assemble and to the passage from a lamellar to a globular structure) favor the formation and spreading of the cracks initiated by the thermic stresses or by cuts or notches such as the grooves produced by the extraction of the cast parts.

Further, the oxidation of the walls of the cracks formed accelerates the spreading thereof.

The liquid metal casting molds are not only subjected to severe conditions of utilization but their manufacture poses many problems. Indeed, in order to be of utility, such molds must have a maximum permissible flexion of no more than 0.5 mm. In respect of hollow cylinders up to 7 meters long, 1.5 meter in diameter and having a thickness of up to I mm., this high precision can only be obtained after straightening and machining operations requiring many precautions. Further, the above-mentioned flexion must not be exceeded in the course of successive thermic cycles imposed on the molds during their utilization.

To summarize the mentioned requirements to determine a choice among grades of existing steels, it is seen that:

a. to support the straightening in the cold state (20-200" C.) without cracks or fractures, the desired steels must not be brittle when cold;

b. to resist wear and scratches or grooves produced by the extraction of the cast parts, these steels must conserve a high hardness in the hot state (usually a Brinell hardness of 250-300 is required thereof);

c. however, to increase the life of the molds, steels are sought in which the spreading of the cracks is as slow as possible and this means selecting steels having high toughness;

d. to reduce the stresses set up by the thermic shock and retard to a certain extent the initiation and spreading of cracks, a dissipation of heat which is as rapid as possible is also desirable; for this purpose, soft iron having the best thermic conductivity and also the smallest coefficient of expansion, which reduces the internal stresses set up by a temperature gradient would be preferable to alloy steels but its low hardness does not allow the use of such an iron.

The compromise between these contradictory requirements, and in particular between high hardness and good thermic conductivity, consequently is difficult to achieve a priori. Applicant has tried to solve this problem experimentally.

The tests first related to known steels classified in accordance with the AFNOR standards and set out in the following Table l:

TABLE I 0 Mn Si Cr Mo Ni V AFNOR reference:

18 CDV4 1.0 0.3

These steels have the following general formula, by weight, in addition to the iron:

Carbon 0.l8-0.30% Manganese about I; Silicon less than 0.4% Chromium none or 0.8-2% Molybdenum 01-03% Nickel none or 1.5%

To obtain the required hardness with such steels, the steel is water quenched or oil quenched and tempered at for instance 600-630C.

The results obtained, although they vary within certain limits, are difiicult to interpret, owing to the difierent conditions of utilization, the different dimensions of the molds, and the. different heat treatments which are the cause of wide dispersions in the yields; this does not allow establishing a formula for steel which is markedly better than the others.

Other tests have led the Applicant to employ tool steels which are particularly renowned for their good thermal resistance. These steels retain at 500-600 C. a good hardness and great rigidity which are qualities that should encourage their use in molds. But their thermic conduction, or, in other words, their heat-dissipating capacity, is distinctly less than that of the steels shown in Table I. This is a major drawback in casting liquid metal in which a rapid cooling of the cast parts is required. Moreover, these steels are very fragile and difficult to machine, even for a given strength.

Finally, the development of weldable low alloy steels employed in the production of sheet metal and in building, has produced a range of water-hardenable and temperable steels whose compositions are often near those of the steels shown in Table I. By way of example, there may be mentioned the steel Tl manufactured by the US. STEEL CORPORATION and having the composition: C 0.10-0.18 percent, Mn 0.5-1.0 percent, Ni 0.5 percent, M0 0.5 percent, Cr 1 percent with traces of B and V. However, although these steels, which have a low alloy content and are weldable, are ductile, their Brinell hardness of 200-270 is insufficient. Air hardened, they have a Brinell hardness of 180-250 in respect of thicknesses of less than 25 mm. which is even lower than in the case of water hardening. Consequently these steels are unsuitable for the purpose in mind.

The object of the invention is to provide an improved mold for casting liquid metal, said mold being of a low alloy steel selected from the range of air-hardenable steels having 0.08-0.28 percent of carbon with additions not exceeding 5 percent of the total, this steel having throughout the thickness of the mold a regular and uniform structure in which at least 60 percent of the carbon is fixed in the form of special carbides, such as those of molybdenum, vanadium and tungsten, the remainder of the carbon being combined in the form of chromiumand manganese-saturated cementite.

The tests carried out by the applicant have shown that up to percent of the carbon can be fixed in the form of carbides, the structure of the mold is very stable and changes but slowly under the effect of repeated heating and cooling between 700 and C.

Preferably, the fineness of the carbides is less than 5 microns and their distance is for instance 20-25 microns.

Another object of the invention is to provide a process for producing the aforementioned improved mold. This process, starting with a low carbon steel, having a composition selected from the following limits by weight in addition to the iron:

Carbon 0.08-0.28k and preferabl 0. l $-0.25% Manganese 0.60-l .40% 0.90-] 40% Silicon 0.20-0.70% 0.30-0.40% Chromium (HO-1.50% LOO-l 25% Molybdenum 0.20- l .00 0.30-l .00'1 Tungsten 0. l -2.00 0.40- l .00% Vanadium 0.00-0.10 0.05-0.07

I the total of these additions not exceeding 5%.

Further optional features and advantages of the invention will appear from the ensuing description, with reference to the accompanying drawings to which the invention is not limited.

In the drawings:

FIG. 1 is a view, with the invention;

FIGS. 2 and 3 are micrograph views of this mold after air quenching and tempering;

FIG. 4 is a perspective view of a test specimen;

FIG. 5 is a diagram of tests carried out on the test specimen shown in FIG. 4;

FIG. 6 is another diagram showing the comparative readings of the residual internal stresses in molds of currently used steel and of steel according to the invention and FIGS. 7 and 8 are micrograph views after oil hardening and tempering.

FIG. 1 shows diagrammatically and only by way of example, a mold A for the centrifugal casting of iron pipes having a male and a socket end.

To produce this or any other like mold, a steel is taken having a composition within the following limits, by weight, in addition to the iron:

parts mit away, of a mold according to Carbon 008-0183: and preferabl (HS-0.25% Manganese 0.60-] .401) 0.90-1 20% Silicon 0.20-0.70l: 0.304140% Chromium 0.90- l .5011 LOO-l .251 Molybdenum 0.20- l .001: 0.30-l .001: Tungsten 0. l0 2.00% 0.80-l .00! Vanadium 0.00-0. I011 0.05-0.071:

the total of these additions not exceeding 5%.

The mold A is preferably formed by a centrifugal casting thereof since this casting results in a fairly rapid solidification and consequently a uniform distribution of the components. This manner of producing the mold is preferred to that of casting from a large ever, this latter manner of proceeding is of utility in particular for large molds.

After rough machining, the shell is preferably air quenched between 950 and 900 C. and then tempered at around 640 C. The Brinell hardness is then between 270 and 300.

FIGS. 2 and 3 show the structure S of the steel thus treated at a magnification of 500 and 6,600 respectively. Outlined at decomposition of the bainite in the course of the tempering, are elongated carbides. The carbides have a very regular distribution and a uniform fineness; their thickness is less than 5 microns with a distance therebetween of 20-25 microns. This uniform structure imparts to the materials great regularity in the flux of heat from the interior towards the exterior of the mold.

Owing to the presence of tungsten, fine carbides are obtained without coarse networks on which the heat treatment would remain without effect. Further, in use, during the casting of the iron in the mold according to the invention, the applicant observed that, owing to the tungsten, the cracks were much slower in appearing. This could be attributed to the aptitude that the steels, of the range employed in accordance with the invention, have of absorbing or damping the thermic stresses set up in the course of the manufacturing cycles by a thermic relaxation phenomenon.

The following tests substantiate this hypothesis.

Test specimens such as that shown in FIG. 4 are formed from a portion of a cylinder 1 through which a conduit 2 exingot followed by forging and boring, howtends in the vicinity of the center corner or edge 3. These test specimens are placed in an induction furnace, the heating of which is concentrated on the edge 3. A circulation of water through the tube or conduit 2 cools the interior of the test specimen. To reproduce the conditions of utilization of a centrifugal casting mold, the specimens are subjected on their edge 3 to thermic cycles including a heating period of about two seconds which raises the surface temperature of the specimen to 650 C. and then a cooling period of about l0 sgconds so as to lower the surface temperature to about C. These thermic cycles or these successive violent heating and energetic cooling produce cracks on the edge 3.

The diagram shown in FIG. 5 gives, as a function of the number n of cycles to which the specimens are subjected, the lengths L of the cracks observed in hundredths of a millimeter. In respect of the steel of a mold according to the invention, the result is shown by the full line I, whereas the curve II in dotted line gives the comparative result for a steel of a known AFNOR type I8CDV4. As can be seen, the specimens corresponding to the steel of the molds according to the invention have cracks on an average only half the length of those of the specimens of the known steel.

In another series of tests, the residual internal stresses of the centrifugal casting mold having an inside diameter of I I2 mm. and a thickness of 12 mm., are measured. The diagram shown in FIG. 6 gives the mean result of these readings. The distance x in millimeters measured from the inner face s (FIG. I) of the mold A are plotted as abscissae and the stresses, measured in decanewtons per square millimeter, are plotted as ordinates y. The positive values in ordinates represent the residual tensile stresses and the negative values the compression stresses. The

curve III in full line represents the behavior of the molds of steel according to the invention, and the curve IV in dotted line represents the behavior of the molds of steel of the known type l8CDV4). As can be seen in the vicinity of the inner face s of the mold, residual tensile stresses exist of the order of 3OdaNlmm. for the mold according to the invention, and SOdaN/mm. for a mold of the known steel. The residual internal stress measured on the inner wall of the mold according to the invention therefore does not reach half that measured on a mold of steel of the known type l8CDV4).

These excellent results are attributed to the presence in the mold according to the invention of refractory carbides of molybdenum, tungsten and vanadium which have fixed at least 60 percent of the total carbon, the rest of the carbon being combined in the form of chromiumand manganesesaturated cementite. These carbides have a very slow coalescence which contributes to the resistance of the steel according to the invention to the thermic cycles. It was owing to this heat treatment of air quenching between 950 and 900 C. followed by a tempering at around 640 C. that it was possible to obtain these carbides.

Although the structure obtained by air hardening is that which gives the best results, it is also possible to contemplate water or oil quenching the steel. In this case, the temperings would be carried out at temperatures 20-40 C. higher than those of the tempering after an air quenching, that is, between 660 and 680 C. However, air quenching is preferred, not only owing to the resultant improved resistance to cracking but also owing to a better shape stability, the water quenching or oil quenching resulting in distortion which necessitates straightening the molds. However this may be, FIGS. 7 and 8, which are similar to FIGS. 2 and 3, show, in the case of oil quenching, the micrographs of the structures 8 obtained. There can be distinguished against a background of even ferrite a lamellae b of carbides which are shorter, finer and more uniformly distributed than in the case of air quenching (compare FIGS. 7 and 8 with FIGS. 2 and 3).

In comparison with the results obtained with steels accordair hardenable in respect of thicknesses of 25-40 mm., their carbon content being less than 0.25 percent.

It is therefore the combination according to the invention of certain compositions of steels and a certain heat treatment which most surprisingly producesthe desired advantages.

Thus, to obtain the penetration of the hardening, traces of boron (0.0020.004 percent) and traces of aluminum (0.0500.150 percent) can be added, the aluminum protecting the boron from oxidation and nitriding.

Nickel can also be added (0.50-1.50 percent) for increasing the depth of the hardening.

Coalescence of the carbides cycles must be slow, whence ing point carbides. For this (0.30-0.50 percent), 0.02-0.05 percent) can under the action of the heating the interest of having high meltpurpose, additions of niobium titanium and zirconium (totalizing be employed for fixing, in addition the carbon which passes from the dissolved state between 1,000-1 ,200E- C. to the precipitated carbide state at around 650-700C. inasmuch as the other requirements of sufficient ductility for a Vickers hardness of 300 are respected.

A steel having the following composition by weight, in addition to the iron could be employed in particular:

Carbon 0.18-0.25'11 Manganese 0.60-l .00% Silicon 0.20-0.70% Chromium 090-1 .50% Molybdenum 0.804130% Tungsten 0L80-l .00% Niobium 0.304150% This last steel analysis is particularly suitable for molds employed in conditions in which the maximum temperature reached on the inner wall exceeds 650 C. during the casting of iron pipes, the transformation point Ac being at 750-780 C. instead of 700-730 C. in respect of steels having smaller contents of special elements such as steels having 0.5 percent of molybdenum, 0.5 percent of tungsten, and 0.1 percent of niobium.

The molds according to the invention are suitable not only for casting liquid iron but also for casting any metal such as aluminum, copper or copper alloys having a melting point under 1,300 C.

It must be understood that the invention is not intended to be limited by the described manners of proceeding and embodiments, which were given solely by way of examples.

Having now described my invention what I claim as new and desire to secure by U.S. Letters Patent is:

l. A centrifugal casting mold for casting liquid metal, said mold being of a low alloy air-hardenable steel consisting essentially of:

carbon 0.08-0.28% by weight manganese 0.60-l.40% by weight silicon 0.20-0.70% by weight chromium 0.90-l.$0% by weight molybdenum 0.20-1.00: by weight tungsten GAO-2.00% by weight vanadium S 0.10% by weight the carbon being combined in the form of chromiumand manganese-saturated cementite, the fineness of the carbides being less than 5 microns and the distance therebetween being about 20-25 microns.

2. A centrifugal casting mold as claimed in claim 1, said steel being of a composition selected from the following proportions by weight Carbon 0. l 543.25% Manganese 0.90-l .40% Silicon 0.30-0.40% Chromium l.00-l.25% Molybdenum 0.30-l .00% Tungsten 0.40-l .00% Vanadium ODS-0.07%

3. A centrifugal casting mold as claimed in claim 1, said steel containing 0.30-0.50 percent of niobium.

4. A centrifugal casting mold as claimed in claim I, said steel containing 002-0115 percentgf titanium and zirconium.

5. A processfor producing a inold for casting liquid metal, said steel having throughout the thickness of the mold a regular and uniform structure in which 60-90 percent of the carbon is fixed in the form of carbides, the remainder of the carbon being combined in the form of chromiumand manganese-saturated cementite, the fineness of the carbides being less than 5 microns and the distance therebetween being about 20-25 microns, said process comprising starting with a low carbon steel having a composition consisting essentially of by weight Carbon 0.08-0.28% Manganese 0.60-l .40% Silicon 0.20-0.70lv Chromium 0.90-l .50% Molybdenum 0.20- l .00% Tungsten 0.10-2.00! Vanadium[0.00-0.l0%] s 0.10% Iron balance.

the total of the additions to the iron not exceeding 5 percent rough machining said mold and subjecting it to a heat treatment of 900 to 950 followed directly by a quenching treatment and thereafter by a tempering treatment.

6. A process as claimed in claim 5, comprising adding 0.50-1.50 percent by weight of nickel to the said low carbon steel composition.

7. A process as claimed in claim 5, comprising adding a trace to the said low carbon steel adding a trace, 0002-0004 percent, of boron and a trace 0.050-0.l50 percent, of aluminum to the said low carbon steel composition.

Carbon 0. l 5-0.25% Manganese 0.90-l.40% Silicon 0.30-0.40k Chromium LOO-1.25% Molybdenum 0.30-1 .001: Tungsten 0.40-l 00% Vanadium ODS-0.07% lron balance,

the total of the additions to the iron not exceeding 5%. 

2. A centrifugal casting mold as claimed in claim 1, said steel being of a composition selected from the following proportions by weight Carbon 0.15-0.25% Manganese 0.90-1.40% Silicon 0.30-0.40% Chromium 1.00-1.25% Molybdenum 0.30-1.00% Tungsten 0.40-1.00% Vanadium 0.05-0.07%
 3. A centrifugal casting mold as claimed in claim 1, said steel containing 0.30- 0.50 percent of niobium.
 4. A centrifugal casting mold as claimed in claim 1, said steel containing 0.02- 0.05 percent of titanium and zirconium.
 5. A process for producing a mold for casting liquid metal, said steel having throughout the thickness of the mold a regular and uniform structure in which 60- 90 percent of the carbon is fixed in the form of carbides, the remainder of the carbon being combined in the form of chromium- and manganese-saturated cementite, the fineness of the carbides being less than 5 microns and the distance therebetween being about 20- 25 microns, said process comprising starting with a low carbon steel having a composition consisting essentially of by weight Carbon 0.08-0.28% Manganese 0.60-1.40% Silicon 0.20-0.70% Chromium 0.90-1.50% Molybdenum 0.20-1.00% Tungsten 0.10-2.00% Vanadium(0.00-0.10%) 0.10% Iron balance, the total of the additions to the iron not exceeding 5 percent rough machining said mold and subjecting it to a heat treatment of 900* to 950* followed directly by a quenching treatment and thereafter by a tempering treatment.
 6. A process as claimed in claim 5, comprIsing adding 0.50- 1.50 percent by weight of nickel to the said low carbon steel composition.
 7. A process as claimed in claim 5, comprising adding to the said low carbon steel a trace, 0.002-0.004 percent, of boron and a trace, 0.050-0.150 percent, of aluminum to the said low carbon steel composition.
 8. A process as claimed in claim 5, comprising adding 0.30- 0.50 percent of niobium to the said low carbon steel composition.
 9. A process as claimed in claim 5, comprising adding 0.02- 0.05 percent of titanium and zirconium to the said low carbon steel composition.
 10. A process as claimed in claim 5, wherein said steel has the following proportions by weight Carbon 0.15-0.25% Manganese 0.90-1.40% Silicon 0.30-0.40% Chromium 1.00-1.25% Molybdenum 0.30-1.00% Tungsten 0.40-1.00% Vanadium 0.05-0.07% Iron balance, the total of the additions to the iron not exceeding 5%. 